Cross talk between beta(1) and alpha(V) integrins: beta(1) affects beta(3) mRNA stability

CASZ1 promotes migration, invasion, and metastasis of lung cancer cells by controlling expression of ITGAV

CASZ1, a zinc finger transcription factor with two isoforms, is known to play important roles in cardiac and neural development. The abnormal expression of CASZ1 is also frequently found in a variety of tumors but has different effects on different tumors; for example, it acts as a tumor suppressor in neuroblastoma but promotes cancer metastasis in ovarian cancer. However, the effect of CASZ1 in lung cancer, the most lethal cancer, remains unclear. Here, we found that the expression of CASZ1 in lung cancer is positively associated with cancer metastasis and poor prognosis. The overexpression of CASZ1b promotes lung cancer cell migration, invasion, and epithelial-mesenchymal transition and is associated with poor prognosis in lung cancer patients. The knockdown of CASZ1 resulted in the suppression of epithelial-mesenchymal transition, migration, and invasion of lung cancer cells and reduced metastasis in vivo. The results of an RNA-sequencing analysis of CASZ1-silenced cells showed that CASZ1 considerably affected the integrin-mediated pathways. CASZ1 bound to the ITGAV promoter and transcriptionally regulated ITGAV expression. Our findings demonstrate that CASZ1 plays an oncogenic role in lung cancer and that CASZ1 promotes lung cancer migration, invasion and metastasis is mediated by ITGAV.

Integrin Crosstalk Contributes to the Complexity of Signalling and Unpredictable Cancer Cell Fates

Integrins are heterodimeric cell surface receptors composed of α and β subunits that control adhesion, proliferation and gene expression. The integrin heterodimer binding to ligand reorganises the cytoskeletal networks and triggers multiple signalling pathways that can cause changes in cell cycle, proliferation, differentiation, survival and motility. In addition, integrins have been identified as targets for many different diseases, including cancer. Integrin crosstalk is a mechanism by which a change in the expression of a certain integrin subunit or the activation of an integrin heterodimer may interfere with the expression and/or activation of other integrin subunit(s) in the very same cell. Here, we review the evidence for integrin crosstalk in a range of cellular systems, with a particular emphasis on cancer. We describe the molecular mechanisms of integrin crosstalk, the effects of cell fate determination, and the contribution of crosstalk to therapeutic outcomes. Our intention is to raise awareness of integrin crosstalk events such that the contribution of the phenomenon can be taken into account when researching the biological or pathophysiological roles of integrins.

Deletion of β1-integrin in collecting duct principal cells leads to tubular injury and renal medullary fibrosis

The renal collecting duct (CD) contains two major cell types, intercalated (ICs) and principal cells (PCs). A previous report showed that deletion of β1-integrin in the entire renal CD causes defective CD morphogenesis resulting in kidney dysfunction. However, subsequent deletion of β1-integrin specifically in ICs and PCs, respectively, did not cause any morphological defects in the CDs. The discrepancy between these studies prompts us to reinvestigate the role of β1-integrin in CD cells, specifically in the PCs. We conditionally deleted β1-integrin in mouse CD PCs using a specific aquaporin-2 (AQP2) promoter Cre-LoxP system. The resulting mutant mice, β-1^f/fAQP2-Cre+, had lower body weight, failed to thrive, and died around 8-12 wk. Their CD tubules were dilated, and some of them contained cellular debris. Increased apoptosis and proliferation of PCs were observed in the dilated CDs. Trichrome staining and electron microscopy revealed the presence of peritubular and interstitial fibrosis that is associated with increased production of extracellular matrix proteins including collagen type IV and fibronectin, as detected by immunoblotting. Further analysis revealed a significantly increased expression of transforming growth factor-β (TGF-β)-induced protein, fibronectin, and TGF-β receptor-1 mRNAs and concomitantly increased phosphorylation of SMAD-2 that indicates the activation of the TGF-β signaling pathway. Therefore, our data reveal that normal expression of β1-integrin in PCs is a critical determinant of CD structural and functional integrity and further support the previously reported critical role of β1-integrin in the development and/or maintenance of the CD structure and function.

αV-class integrins exert dual roles on α5β1 integrins to strengthen adhesion to fibronectin

Upon binding to the extracellular matrix protein, fibronectin, αV-class and α5β1 integrins trigger the recruitment of large protein assemblies and strengthen cell adhesion. Both integrin classes have been functionally specified, however their specific roles in immediate phases of cell attachment remain uncharacterized. Here, we quantify the adhesion of αV-class and/or α5β1 integrins expressing fibroblasts initiating attachment to fibronectin (≤120 s) by single-cell force spectroscopy. Our data reveals that αV-class integrins outcompete α5β1 integrins. Once engaged, αV-class integrins signal to α5β1 integrins to establish additional adhesion sites to fibronectin, away from those formed by αV-class integrins. This crosstalk, which strengthens cell adhesion, induces α5β1 integrin clustering by RhoA/ROCK/myosin-II and Arp2/3-mediated signalling, whereas overall cell adhesion depends on formins. The dual role of both fibronectin-binding integrin classes commencing with an initial competition followed by a cooperative crosstalk appears to be a basic cellular mechanism in assembling focal adhesions to the extracellular matrix.

microRNA miR-142-3p Inhibits Breast Cancer Cell Invasiveness by Synchronous Targeting of WASL, Integrin Alpha V, and Additional Cytoskeletal Elements

MicroRNAs (miRNAs, micro ribonucleic acids) are pivotal post-transcriptional regulators of gene expression. These endogenous small non-coding RNAs play significant roles in tumorigenesis and tumor progression. miR-142-3p expression is dysregulated in several breast cancer subtypes. We aimed at investigating the role of miR-142-3p in breast cancer cell invasiveness. Supported by transcriptomic Affymetrix array analysis and confirmatory investigations at the mRNA and protein level, we demonstrate that overexpression of miR-142-3p in MDA-MB-231, MDA-MB-468 and MCF-7 breast cancer cells leads to downregulation of WASL (Wiskott-Aldrich syndrome-like, protein: N-WASP), Integrin-αV, RAC1, and CFL2, molecules implicated in cytoskeletal regulation and cell motility. ROCK2, IL6ST, KLF4, PGRMC2 and ADCY9 were identified as additional targets in a subset of cell lines. Decreased Matrigel invasiveness was associated with the miR-142-3p-induced expression changes. Confocal immunofluorescence microscopy, nanoscale atomic force microscopy and digital holographic microscopy revealed a change in cell morphology as well as a reduced cell volume and size. A more cortical actin distribution and a loss of membrane protrusions were observed in cells overexpressing miR-142-3p. Luciferase activation assays confirmed direct miR-142-3p-dependent regulation of the 3’-untranslated region of ITGAV and WASL. siRNA-mediated depletion of ITGAV and WASL resulted in a significant reduction of cellular invasiveness, highlighting the contribution of these factors to the miRNA-dependent invasion phenotype. While knockdown of WASL significantly reduced the number of membrane protrusions compared to controls, knockdown of ITGAV resulted in a decreased cell volume, indicating differential contributions of these factors to the miR-142-3p-induced phenotype. Our data identify WASL, ITGAV and several additional cytoskeleton-associated molecules as novel invasion-promoting targets of miR-142-3p in breast cancer.

In PC3 prostate cancer cells ephrin receptors crosstalk to β1-integrins to strengthen adhesion to collagen type I

Eph receptor (Eph) and ephrin signaling can play central roles in prostate cancer and other cancer types. Exposed to ephrin-A1 PC3 prostate cancer cells alter adhesion to extracellular matrix (ECM) proteins. However, whether PC3 cells increase or reduce adhesion, and by which mechanisms they change adhesion to the ECM remains to be characterized. Here, we assay how ephrin-A1 stimulates PC3 cells to adhere to ECM proteins using single-cell force spectroscopy. We find that PC3 cells binding to immobilized ephrin-A1 but not to solubilized ephrin-A1 specifically strengthen adhesion to collagen I. This Eph-ephrin-A1 signaling, which we suppose is based on mechanotransduction, stimulates β₁-subunit containing integrin adhesion via the protein kinase Akt and the guanine nucleotide-exchange factor cytohesin. Inhibiting the small GTPases, Rap1 or Rac1, generally lowered adhesion of PC3 prostate cancer cells. Our finding suggests a mechanism by which PC3 prostate cancer cells exposed to ephrins crosstalk to β₁-integrins and preferably metastasize in bone, a collagen I rich tissue.

Nanoparticle-formulated siRNA targeting integrins inhibits hepatocellular carcinoma progression in mice

Integrins play an important role during development, regulating cell differentiation, proliferation and survival. Here we show that knockdown of integrin subunits slows down the progression of hepatocellular carcinoma (HCC). Using nanoparticulate delivery of short interfering RNAs targeting β1 and αv integrin subunits we downregulate all integrin receptors in hepatocytes. Short-term integrin knockdown (two weeks) does not cause apparent structural or functional perturbations of normal liver tissue. Alterations in liver morphology accumulate upon sustained integrin downregulation (seven weeks). The integrin knockdown leads to significant retardation of HCC progression, reducing proliferation and increasing tumour cell death. This tumour retardation is accompanied by reduced activation of MET oncogene as well as expression of its mature form on the cell surface. Our data suggest that transformed proliferating cells from HCC are more sensitive to knockdown of integrins than normal quiescent hepatocytes, highlighting the potential of siRNA-mediated inhibition of integrins as an anti-cancer therapeutic approach.

β1 integrin inhibition elicits a prometastatic switch through the TGFβ-miR-200-ZEB network in E-cadherin-positive triple-negative breast cancer

Interactions with the extracellular matrix (ECM) through integrin adhesion receptors provide cancer cells with physical and chemical cues that act together with growth factors to support survival and proliferation. Antagonists that target integrins containing the β1 subunit inhibit tumor growth and sensitize cells to irradiation or cytotoxic chemotherapy in preclinical breast cancer models and are under clinical investigation. We found that the loss of β1 integrins attenuated breast tumor growth but markedly enhanced tumor cell dissemination to the lungs. When cultured in three-dimensional ECM scaffolds, antibodies that blocked β1 integrin function or knockdown of β1 switched the migratory behavior of human and mouse E-cadherin-positive triple-negative breast cancer (TNBC) cells from collective to single cell movement. This switch involved activation of the transforming growth factor-β (TGFβ) signaling network that led to a shift in the balance between miR-200 microRNAs and the transcription factor zinc finger E-box-binding homeobox 2 (ZEB2), resulting in suppressed transcription of the gene encoding E-cadherin. Reducing the abundance of a TGFβ receptor, restoring the ZEB/miR-200 balance, or increasing the abundance of E-cadherin reestablished cohesion in β1 integrin-deficient cells and reduced dissemination to the lungs without affecting growth of the primary tumor. These findings reveal that β1 integrins control a signaling network that promotes an epithelial phenotype and suppresses dissemination and indicate that targeting β1 integrins may have undesirable effects in TNBC.

Combined pulldown and time-lapse microscopy studies for determining the role of Rap1 in the crosstalk between integrins and cadherins

The coordinate modulation of the cellular functions of cadherins and integrins plays an essential role in fundamental physiological and pathological processes, including morphogenesis, tissue differentiation and renewal, wound healing, immune surveillance, inflammatory response, tumor progression, and metastasis. Recent findings state the molecular mechanisms underlying the fine-balanced relationship between cadherins and integrins. In particular, some of the novel results recently obtained raise the possibility of a pivotal role for the small GTPase Rap1 in the functional crosstalk between cadherins and integrins. Considering the importance of the molecular signalling triggered by Rap1, here we provide protocols to study this small GTPase in signalling pathways involving cadherins and integrins.

Molecular composition and function of integrin-based collagen glues-introducing COLINBRIs

BACKGROUND

Despite detailed knowledge about the structure and signaling properties of individual collagen receptors, much remains to be learned about how these receptors participate in linking cells to fibrillar collagen matrices in tissues. In addition to collagen-binding integrins, a group of proteins with affinity both for fibrillar collagens and integrins link these two protein families together. We have introduced the name COLINBRI (COLlagen INtegrin BRIdging) for this set of molecules. Whereas collagens are the major building blocks in tissues and defects in these structural proteins have severe consequences for tissue integrity, the mild phenotypes of the integrin type of collagen receptors have raised questions about their importance in tissue biology and pathology.

SCOPE OF REVIEW

We will discuss the two types of cell linkages to fibrillar collagen (direct- versus indirect COLINBRI-mediated) and discuss how the parallel existence of direct and indirect linkages to collagens may ensure tissue integrity.

MAJOR CONCLUSIONS

The observed mild phenotypes of mice deficient in collagen-binding integrins and the relatively restricted availability of integrin-binding sequences in mature fibrillar collagen matrices support the existence of indirect collagen-binding mechanisms in parallel with direct collagen binding in vivo.

GENERAL SIGNIFICANCE

A continued focus on understanding the molecular details of cell adhesion mechanisms to collagens will be important and will benefit our understanding of diseases like tissue- and tumor fibrosis where collagen dynamics are disturbed. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

The role of integrins in the trabecular meshwork

Integrins are a family of heterodimeric transmembrane receptors that mediate adhesion to the extracellular matrix (ECM). However, integrins are not just adhesion receptors. They can act as "bidirectional signal transducers" that coordinate a large number of cellular activities in response to the extracellular environment and intracellular signaling events. Among the activities regulated by integrins are cell adhesion, assembly of the ECM, growth factor signaling, apoptosis, organization of the cytoskeleton, and cytoskeleton-mediated processes such as contraction, endocytosis, and phagocytosis. Integrins regulate these activities through a complex network of intracellular signaling kinases and adaptor proteins that associate with the transmembrane and cytoplasmic domains of the integrin subunits. In this review, we will discuss how some of the known integrin-mediated activities can control the function of the trabecular meshwork. We will also discuss how integrin activity is a tightly regulated process that involves conformation changes within the heterodimer which are mediated by specific integrin-binding proteins.

αV-integrins are required for mechanotransduction in MDCK epithelial cells

The properties of epithelial cells within tissues are regulated by their immediate microenvironment, which consists of neighboring cells and the extracellular matrix (ECM). Integrin heterodimers orchestrate dynamic assembly and disassembly of cell-ECM connections and thereby convey biochemical and mechanical information from the ECM into cells. However, the specific contributions and functional hierarchy between different integrin heterodimers in the regulation of focal adhesion dynamics in epithelial cells are incompletely understood. Here, we have studied the functions of RGD-binding αV-integrins in a Madin Darby Canine Kidney (MDCK) cell model and found that αV-integrins regulate the maturation of focal adhesions (FAs) and cell spreading. αV-integrin-deficient MDCK cells bound collagen I (Col I) substrate via α2β1-integrins but failed to efficiently recruit FA components such as talin, focal adhesion kinase (FAK), vinculin and integrin-linked kinase (ILK). The apparent inability to mature α2β1-integrin-mediated FAs and link them to cellular actin cytoskeleton led to disrupted mechanotransduction in αV-integrin deficient cells seeded onto Col I substrate.

Rad9 protein contributes to prostate tumor progression by promoting cell migration and anoikis resistance

Rad9 as part of the Rad9-Hus1-Rad1 complex is known to participate in cell cycle checkpoint activation and DNA repair. However, Rad9 can act as a sequence-specific transcription factor, modulating expression of a number of genes. Importantly, Rad9 is up-regulated in prostate cancer cell lines and clinical specimens. Its expression correlates positively with advanced stage tumors and its down-regulation reduces tumor burden in mice. We show here that transient down-regulation of Rad9 by RNA interference reduces DU145 and PC3 prostate cancer cell proliferation and survival in vitro. In addition, transient or stable down-regulation of Rad9 impairs migration and invasion of the cells. Moreover, stable reduction of Rad9 renders DU145 cell growth anchorage-dependent. It also decreases expression of integrin β1 protein and sensitizes DU145 and LNCaP cells to anoikis and impairs Akt activation. On the other hand, stable expression of Mrad9, the mouse homolog, in DU145/shRNA Rad9 cells restores migration, invasion, anchorage-independent growth, integrin β1 expression, and anoikis resistance with a concomitant elevation of Akt activation. We thus demonstrate for the first time that Rad9 contributes to prostate tumorigenesis by increasing not only tumor proliferation and survival but also tumor migration and invasion, anoikis resistance, and anchorage-independent growth.

Expression and function of cell adhesion molecules during neural crest migration

Neural crest cells are highly migratory cells that give rise to many derivatives including peripheral ganglia, craniofacial structures and melanocytes. Neural crest cells migrate along defined pathways to their target sites, interacting with each other and their environment as they migrate. Cell adhesion molecules are critical during this process. In this review we discuss the expression and function of cell adhesion molecules during the process of neural crest migration, in particular cadherins, integrins, members of the immunoglobulin superfamily of cell adhesion molecules, and the proteolytic enzymes that cleave these cell adhesion molecules. The expression and function of these cell adhesion molecules and proteases are compared across neural crest emigrating from different axial levels, and across different species of vertebrates.

RGD-Binding Integrins in Prostate Cancer: Expression Patterns and Therapeutic Prospects against Bone Metastasis

Prostate cancer is the third leading cause of male cancer deaths in the developed world. The current lack of highly specific detection methods and efficient therapeutic agents for advanced disease have been identified as problems requiring further research. The integrins play a vital role in the cross-talk between the cell and extracellular matrix, enhancing the growth, migration, invasion and metastasis of cancer cells. Progression and metastasis of prostate adenocarcinoma is strongly associated with changes in integrin expression, notably abnormal expression and activation of the β₃ integrins in tumour cells, which promotes haematogenous spread and tumour growth in bone. As such, influencing integrin cell expression and function using targeted therapeutics represents a potential treatment for bone metastasis, the most common and debilitating complication of advanced prostate cancer. In this review, we highlight the multiple ways in which RGD-binding integrins contribute to prostate cancer progression and metastasis, and identify the rationale for development of multi-integrin antagonists targeting the RGD-binding subfamily as molecularly targeted agents for its treatment.

Molecular Crosstalk between Integrins and Cadherins: Do Reactive Oxygen Species Set the Talk?

The coordinate modulation of the cellular functions of cadherins and integrins plays an essential role in fundamental physiological and pathological processes, including morphogenesis, tissue differentiation and renewal, wound healing, immune surveillance, inflammatory response, tumor progression, and metastasis. However, the molecular mechanisms underlying the fine-tuned functional communication between cadherins and integrins are still elusive. This paper focuses on recent findings towards the involvement of reactive oxygen species (ROS) in the regulation of cell adhesion and signal transduction functions of integrins and cadherins, pointing to ROS as emerging strong candidates for modulating the molecular crosstalk between cell-matrix and cell-cell adhesion receptors.

Transfection of human platelets with short interfering RNA

Platelets contain mRNAs and are capable of translating mRNA into protein, and it has been previously demonstrated that platelets increase their levels of integrin β3 overtime while in blood bank storage conditions. We are unaware of prior attempts to introduce nucleic acids into platelets. Considering the potential clinical and research utility of manipulating platelet gene expression, we tested whether small interfering RNAs (siRNAs) could be transfected into normal human platelets. Multiple conditions were tested, including lipofectamine versus electroporation, different amounts of siRNA, the effect of different buffers and the presence of plasma during transfection, and the time for optimal siRNA incorporation after transfection. Using flow cytometry to assess transfection efficiency, we found that optimal transfection was obtained using lipofectamine, washed platelets, and 400 pmoles siRNA. Cell sorting of transfected platelets suggested that the incorporated siRNA was able to knockdown the level of a targeted mRNA. This is the first ever demonstration that nucleic acids can be introduced directly into platelets, and offers proof of concept for manipulating gene expression in platelets by nonviral methods. Future technical improvements may permit improving the quality and/or lifespan of stored human platelets.

R-Ras interacts with filamin a to maintain endothelial barrier function

The molecular mechanisms regulating vascular barrier integrity remain incompletely elucidated. We have previously reported an association between the GTPase R-Ras and repeat 3 of Filamin A (FLNa). Loss of FLNa has been linked to increased vascular permeability. We sought to determine whether FLNa's association with R-Ras affects endothelial barrier function. We report that in endothelial cells endogenous R-Ras interacts with endogenous FLNa as determined by co-immunoprecipitations and pulldowns with the FLNa-GST fusion protein repeats 1-10. Deletion of FLNa repeat 3 (FLNaΔ3) abrogated this interaction. In these cells FLNa and R-Ras co-localize at the plasma membrane. Knockdown of R-Ras and/or FLNa by siRNA promotes vascular permeability, as determined by TransEndothelial Electrical Resistance and FITC-dextran transwell assays. Re-expression of FLNa restored endothelial barrier function in cells lacking FLNa whereas re-expression of FLNaΔ3 did not. Immunostaining for VE-Cadherin in cells with knocked down R-Ras and FLNa demonstrated a disorganization of VE-Cadherin at adherens junctions. Loss of R-Ras and FLNa or blocking R-Ras function via GGTI-2133, a selective R-Ras inhibitor, induced vascular permeability and increased phosphorylation of VE-Cadherin (Y731) and Src (Y416). Expression of dominant negative R-Ras promoted vascular permeability that was blocked by the Src inhibitor PP2. These findings demonstrate that maintaining endothelial barrier function is dependent upon active R-Ras and association between R-Ras and FLNa and that loss of this interaction promotes VE-Cadherin phosphorylation and changes in downstream effectors that lead to endothelial leakiness.

RNA interference-mediated knockdown of CD49e (α5 integrin chain) in human thymic epithelial cells modulates the expression of multiple genes and decreases thymocyte adhesion

BACKGROUND

The thymus is a central lymphoid organ, in which bone marrow-derived T cell precursors undergo a complex process of maturation. Developing thymocytes interact with thymic microenvironment in a defined spatial order. A component of thymic microenvironment, the thymic epithelial cells, is crucial for the maturation of T-lymphocytes through cell-cell contact, cell matrix interactions and secretory of cytokines/chemokines. There is evidence that extracellular matrix molecules play a fundamental role in guiding differentiating thymocytes in both cortical and medullary regions of the thymic lobules. The interaction between the integrin α5β1 (CD49e/CD29; VLA-5) and fibronectin is relevant for thymocyte adhesion and migration within the thymic tissue. Our previous results have shown that adhesion of thymocytes to cultured TEC line is enhanced in the presence of fibronectin, and can be blocked with anti-VLA-5 antibody.

RESULTS

Herein, we studied the role of CD49e expressed by the human thymic epithelium. For this purpose we knocked down the CD49e by means of RNA interference. This procedure resulted in the modulation of more than 100 genes, some of them coding for other proteins also involved in adhesion of thymocytes; others related to signaling pathways triggered after integrin activation, or even involved in the control of F-actin stress fiber formation. Functionally, we demonstrated that disruption of VLA-5 in human TEC by CD49e-siRNA-induced gene knockdown decreased the ability of TEC to promote thymocyte adhesion. Such a decrease comprised all CD4/CD8-defined thymocyte subsets.

CONCLUSION

Conceptually, our findings unravel the complexity of gene regulation, as regards key genes involved in the heterocellular cell adhesion between developing thymocytes and the major component of the thymic microenvironment, an interaction that is a mandatory event for proper intrathymic T cell differentiation.

ADAM2 interactions with mouse eggs and cell lines expressing α4/α9 (ITGA4/ITGA9) integrins: implications for integrin-based adhesion and fertilization

BACKGROUND

Integrins are heterodimeric cell adhesion molecules, with 18 α (ITGA) and eight β (ITGB) subunits forming 24 heterodimers classified into five families. Certain integrins, especially the α(4)/α(9) (ITGA4/ITGA9) family, interact with members of the ADAM (a disintegrin and metalloprotease) family. ADAM2 is among the better characterized and also of interest because of its role in sperm function. Having shown that ITGA9 on mouse eggs participates in mouse sperm-egg interactions, we sought to characterize ITGA4/ITGA9-ADAM2 interactions.

METHODOLOGY/PRINCIPAL FINDINGS

An anti-β(1)/ITGB1 function-blocking antibody that reduces sperm-egg binding significantly inhibited ADAM2 binding to mouse eggs. Analysis of integrin subunit expression indicates that mouse eggs could express at least ten different integrins, five in the RGD-binding family, two in the laminin-binding family, two in the collagen-binding family, and ITGA9-ITGB1. Adhesion assays to characterize ADAM2 interactions with ITGA4/ITGA9 family members produced the surprising result that RPMI 8866 cell adhesion to ADAM2 was inhibited by an anti-ITGA9 antibody, noteworthy because ITGA9 has only been reported to dimerize with ITGB1, and RPMI 8866 cells lack detectable ITGB1. Antibody and siRNA studies demonstrate that ITGB7 is the β subunit contributing to RPMI 8866 adhesion to ADAM2.

CONCLUSIONS/SIGNIFICANCE

These data indicate that a novel integrin α-β combination, ITGA9-ITGB7 (α(9)β(7)), in RPMI 8866 cells functions as a binding partner for ADAM2. ITGA9 had previously only been reported to dimerize with ITGB1. Although ITGA9-ITGB7 is unlikely to be a widely expressed integrin and appears to be the result of "compensatory dimerization" occurring in the context of little/no ITGB1 expression, the data indicate that ITGA9-ITGB7 functions as an ADAM binding partner in certain cellular contexts, with implications for mammalian fertilization and integrin function.

TIMP-1 interaction with αvβ3 integrin confers resistance to human osteosarcoma cell line MG-63 against TNF-α-induced apoptosis

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine affecting diverse cellular responses. TNF-α is cytotoxic in many systems, but it can also act as an anti-apoptotic signal to promote cell survival pathways activated through integrins and extracellular matrix components. This is particularly evident in cancer cells. To unravel the basis of resistance to TNF-α-induced apoptosis, human osteosarcoma MG-63 cell line was used. Our data showed that resistance to apoptosis was accompanied by high levels of TIMP-1 expression in part mediated by NF-κB activation, whereas under apoptotic conditions, in the presence of cycloheximide (CHX), TIMP-1 and αvβ3 integrin protein levels were significantly reduced. Silencing TIMP-1 using siRNA led to increased apoptosis following treatment with TNF-α, whereas exogenously-added recombinant TIMP-1 reduced the extent of apoptosis. Immunoprecipitation and confocal microscopy experiments demonstrated that TIMP-1 interacted with αvβ3 integrins. The biological role of this interaction was revealed by the use of echistatin, an antagonist of αvβ3 integrin. In the presence of echistatin, decreased protection against apoptosis by recombinant TIMP-1 was observed.

Loss of beta1-integrin enhances TGF-beta1-induced collagen expression in epithelial cells via increased alphavbeta3-integrin and Rac1 activity

Transforming growth factor β (TGF-β) promotes tissue fibrosis via the receptor-specific Smad pathway and non-canonical pathways. We recently reported that TGF-β1-stimulated collagen expression by cultured kidney cells requires integrin-dependent activation of focal adhesion kinase (FAK) and consequent ERK MAP kinase activity leading to Smad3 linker region phosphorylation. Here, we defined a role for αvβ3-integrin in this non-canonical pathway. A human kidney tubular cell line in which β1-integrin was knocked down (β1-k/d) demonstrated enhanced type I collagen mRNA expression and promoter activity. A second shRNA to either αv-integrin or β3-integrin, but not to another αv-binding partner, β6-integrin, abrogated the enhanced COL1A2 promoter activity in β1-k/d cells. Although αvβ3-integrin surface expression levels were not different, αvβ3-integrins colocalized with sites of focal adhesion significantly more in β1-k/d cells, and activated αvβ3-integrin was detected only in β1-k/d cells. Further, the collagen response was decreased by a function-blocking antibody or a peptide inhibitor of αvβ3-integrin. In cells lacking αvβ3-integrin, the responses were attenuated, whereas the response was enhanced in αvβ3-overexpressing cells. Rac1 and ERK, previously defined mediators for this non-canonical pathway, showed increased activities in β1-k/d cells. Finally, inhibition of αvβ3-integrin decreased Rac1 activity and COL1A2 promoter activity in β1-k/d cells. Together, our results indicate that decreasing β1 chain causes αvβ3-integrin to become functionally dominant and promotes renal cell fibrogenesis via Rac1-mediated ERK activity.

iso-DGR sequences do not mediate binding of fibronectin N-terminal modules to adherent fibronectin-null fibroblasts

Fibronectin (FN) without an RGD sequence (FN-RGE), and thus lacking the principal binding site for alpha5beta1 integrin, is deposited into the extracellular matrix of mouse embryos. Spontaneous conversion of (263)NGR and/or (501)NGR to iso-DGR possibly explains this enigma, i.e. ligation of iso-DGR by alphavbeta3 integrin may allow cells to assemble FN. Partial modification of (263)NGR to DGR or iso-DGR was detected in purified plasma FN by mass spectrometry. To test functions of the conversion, one or both NGR sequences were mutated to QGR in recombinant N-terminal 70-kDa construct of FN (70K), full-length FN, or FN-RGE. The mutations did not affect the binding of soluble 70K to already adherent fibroblasts or the ability of soluble 70K to compete with non-mutant FN or FN-RGE for binding to FN assembly sites. Non-mutant FN and FN-N263Q/N501Q with both NGRs mutated to QGRs were assembled equally well by adherent fibroblasts. FN-RGE and FN-RGE-N263Q/N501Q were also assembled equally well. Although substrate-bound 70K mediated cell adhesion in the presence of 1 mm Mn(2+) by a mechanism that was inhibited by cyclic RGD peptide, the peptide did not inhibit 70K binding to cell surface. Mutations of the NGR sequences had no effect on Mn(2+)-enhanced cell adhesion to adsorbed 70K but caused a decrease in cell adhesion to reduced and alkylated 70K. These results demonstrate that iso-DGR sequences spontaneously converted from NGR are cryptic and do not mediate the interaction of the 70K region of FN with the cell surface during FN assembly.

Transdominant regulation of integrin function: mechanisms of crosstalk

Transdominant inhibition of integrins or integrin-integrin crosstalk is an important regulator of integrin ligand binding and subsequent signaling events that control a variety of cell functions in many tissues. Here we discuss examples of integrin crosstalk and detail our current understanding of the molecular mechanisms that are involved in this receptor phenomenon. The cytoskeleton associated protein talin is a key regulator of integrin crosstalk. We describe how the interaction of talin and the cytoplasmic tail of beta integrin is controlled and how competitive inhibitors of this binding play a role in integrin crosstalk. We conclude with a discussion of how integrin crosstalk impacts the interpretation of integrin inhibitor and knockdown studies in both the laboratory and clinical setting.

Dissecting the role of integrin subunits alpha 2 and beta 3 in rotavirus cell entry by RNA silencing

Several cell surface molecules have been implicated in rotavirus cell entry, however, their individual relevance during this process is unknown. In this work, the expression of integrins alpha2, beta2, and alpha v beta 3, the heat shock cognate protein 70, and of ganglioside GM1 in different cell lines of human and simian origin was correlated with the infectivity of four rotavirus strains. We observed that different combinations of receptor expression correlated with the infectivity of rotavirus strains, suggesting that the participation of several receptors is important for rotavirus infection. To characterize the relevance of integrins alpha2 and alpha v beta 3 in more detail, their expression was silenced using RNA interference. About 80% decrease in the cell content of integrins resulted in 15-30% decrease of infectivity of strains RRV and Wa when measured by a focus-forming assay, while there was no decrease of infectivity when measured by flow cytometry in integrin-deficient cells. Altogether these data suggest that integrins alpha2 and alpha v beta 3 do not play a major role in the rotavirus entry process.

Integrin alphavbeta1 promotes infection by human metapneumovirus

Human metapneumovirus (hMPV) is a recently described paramyxovirus that causes lower respiratory infections in children and adults worldwide. The hMPV fusion (F) protein is a membrane-anchored glycoprotein and major protective antigen. All hMPV F protein sequences determined to date contain an Arg-Gly-Asp (RGD) sequence, suggesting that F engages RGD-binding integrins to mediate cell entry. The divalent cation chelator EDTA, which disrupts heterodimeric integrin interactions, inhibits infectivity of hMPV but not the closely related respiratory syncytial virus (RSV), which lacks an RGD motif. Function-blocking antibodies specific for alphavbeta1 integrin inhibit infectivity of hMPV but not RSV. Transfection of nonpermissive cells with alphav or beta1 cDNAs confers hMPV infectivity, whereas reduction of alphav and beta1 integrin expression by siRNA inhibits hMPV infection. Recombinant hMPV F protein binds to cells, whereas Arg-Gly-Glu (RGE)-mutant F protein does not. These data suggest that alphavbeta1 integrin is a functional receptor for hMPV.

Reduction of mouse egg surface integrin alpha9 subunit (ITGA9) reduces the egg's ability to support sperm-egg binding and fusion

The involvement of egg integrins in mammalian sperm-egg interactions has been controversial, with data from integrin inhibitor studies contrasting with evidence from knockouts showing that specific integrin subunits are not essential for fertility. An alpha(4)/alpha(9) (ITGA4/ITGA9) integrin subfamily member has been implicated in fertilization but not extensively examined, so we tested the following three hypotheses: 1) an ITGA4/ITGA9 integrin participates in sperm-egg interactions, 2) short-term acute knockdown by RNA interference of integrin subunits would result in a fertilization phenotype differing from that of chronic depletion via knockout, and 3) detection of a fertilization phenotype is sensitive to in vitro fertilization (IVF) assay conditions. We show that mouse and human eggs express the alpha(9) integrin subunit (ITGA9). RNA interference-mediated knockdown resulted in reduced levels of Itga9 mRNA and surface protein in mouse eggs. RNA interference attempts to knockdown ITGA9's likely beta partner, beta(1) (ITGB1), resulted in reduced Itgb1 mRNA but no reduction in ITGB1 surface protein. Therefore, studies using a function-blocking anti-ITGB1 antibody tested the hypothesis that ITGB1 participates in gamete interactions. Analyses of sperm-egg interactions with Itga9-knockdown eggs and anti-ITGB1 antibody-treated eggs in IVF assays using specific sperm:egg ratios revealed the following: 1) a reduction, but not complete loss, of sperm-egg binding and fusion was observed and 2) the reduction of sperm-egg binding and fusion was not detected in inseminations with high sperm:egg ratios. These data demonstrate that ITGA9 and ITGB1 participate in sperm-egg interactions but clearly are not the only molecules involved. This also shows that careful design of IVF parameters allows detection of deficiencies in gamete interactions.

Integrin trafficking regulated by Rab21 is necessary for cytokinesis

Adherent cells undergo remarkable changes in shape during cell division. However, the functional interplay between cell adhesion turnover and the mitotic machinery is poorly understood. The endo/exocytic trafficking of integrins is regulated by the small GTPase Rab21, which associates with several integrin alpha subunits. Here, we show that targeted trafficking of integrins to and from the cleavage furrow is required for successful cytokinesis, and that this is regulated by Rab21. Rab21 activity, integrin-Rab21 association, and integrin endocytosis are all necessary for normal cytokinesis, which becomes impaired when integrin-mediated adhesion at the cleavage furrow fails. We also describe a chromosomal deletion and loss of Rab21 gene expression in human cancer, which leads to the accumulation of multinucleate cells. Importantly, reintroduction of Rab21 rescued this phenotype. In conclusion, Rab21-regulated integrin trafficking is essential for normal cell division, and its defects may contribute to multinucleation and genomic instability, which are hallmarks of cancer.

Extended interaction of beta1 integrin subunit-deficient cells (GD25) with surfaces modified with fibronectin-derived peptides: Culture optimization, adhesion and cytokine panel studies

The modification of biomaterials with extracellular matrix-mimicking factors is a common technique used to influence the cellular response through integrin-mediated signaling. The inherent limitations of antibody-inhibition studies necessitate the use of complementary methods to block integrin function to confirm cell-surface interaction. In this study, we employed a beta1 integrin-deficient cell line, GD25, to investigate the role of beta1 subunit in cell adhesion and subsequent cytokine (granulocyte macrophage colony stimulating factor; interleukin (IL)-1alpha; IL-1beta; IL-6; monocyte chemoattractant protein-1; regulated upon activation, normal T-cell expressed, and secreted; tumor necrosis factor-alpha) release kinetics in the presence of tissue culture polystyrene (TCPS) and semi-interpenetrating polymer networks (sIPN) modified with fibronectin (FN)-mimic peptides (RGD, PHSRN). Culture conditions (i.e. seeding density, medium, serum supplementation) were optimized for long-term observation. Differences in cell adhesion, cell viability and cytokine release behavior were dependent on the presence of the beta1 integrin subunit, FN, sIPN cast method and peptide identity. By comparing two complementary techniques for assaying integrin function, we observed both similarities (i.e. decreased adhesion to FN-absorbed TCPS and increased IL-1beta release at 96h) and differences (i.e. no difference in adhesion or IL-1beta release in the presence of different sIPN surfaces) when the function of the beta1 subunit was blocked in cell adhesion and signaling in the presence of biomaterials.

Fibrillin-1 microfibril deposition is dependent on fibronectin assembly

Newly deposited microfibrils strongly colocalise with fibronectin in primary fibroblasts. Microfibril formation is grossly inhibited by fibronectin depletion, but rescued by supplementation with exogenous cellular fibronectin. As integrin receptors are key determinants of fibronectin assembly, we investigated whether they also influenced microfibril deposition. Analysis of beta1-integrin-receptor-null fibroblasts, blockage of cell surface integrin receptors that regulate fibronectin assembly and disruption of Rho kinase all result in suppressed deposition of both fibronectin and microfibrils. Antibody activation of beta1 integrins in fibronectin-depleted cultures is insufficient to rescue microfibril assembly. In fibronectin(RGE/RGE) mutant mouse fibroblast cultures, which do not engage alpha5beta1 integrin, extracellular assembly of both fibronectin and microfibrils is markedly reduced. Thus, pericellular microfibril assembly is regulated by fibronectin fibrillogenesis.

A Toll-like receptor 2-integrin beta3 complex senses bacterial lipopeptides via vitronectin

Toll-like receptor 2 (TLR2) initiates inflammation in response to bacterial lipopeptide (BLP). However, the molecular mechanisms enabling the detection of BLP by TLR2 are unknown. Here we investigated the interaction of BLP with human serum proteins and identified vitronectin as a BLP-recognition molecule. Vitronectin and its receptor, integrin beta(3), were required for BLP-induced TLR2-mediated activation of human monocytes. Furthermore, monocytes from patients with Glanzmann thrombasthenia, which lack integrin beta(3), were completely unresponsive to BLP. In addition, integrin beta(3) formed a complex with TLR2 and this complex dissociated after BLP stimulation. Notably, vitronectin and integrin beta(3) coordinated responses to other TLR2 agonists such as lipoteichoic acid and zymosan. Our findings show that vitronectin and integrin beta(3) contribute to the initiation of TLR2 responses.

Rho GTPases mediated integrin alpha v beta 3 activation in sphingosine-1-phosphate stimulated chemotaxis of endothelial cells

Integrins, a family of transmembrane heterodimeric polypeptides, mediate various biological responses including cell adhesion and migration. In this report, we show that sphingosine-1-phosphate (S1P) activates integrin alpha v beta 3 in endothelial cells (ECs) via the sphingosine-1-phosphate receptor subtype 1 (S1P1)-mediated signaling pathway. S1P treatment results in the activation of integrin alpha v beta 3 in the lamellipodia region of ECs, suggesting that integrin alpha v beta 3 plays a critical role in the S1P-stimulated chemotactic response of ECs. Indeed, S1P treatment induces the association of focal adhesion kinase (FAK) and cytoskeletal proteins with integrin alpha v beta 3, the ligation of alpha v and beta 3 subunits, as well as enhances endothelial migration on vitronectin-coated substrata. Knockdown endothelial S1P1 receptor, treatments with pertussis toxin or dominant-negative-Rho family GTPases abrogates the S1P-induced integrin alpha v beta 3 activation in ECs. Consequently, these treatments markedly inhibit the S1P-induced endothelial migratory response on vitronectin-coated substrata. Collectively, these data indicate that the S1P-mediated signaling via the S1P1/Gi/Rho GTPases pathway activates integrin alpha v beta 3, which is indispensable for S1P-stimulated chemotactic response of ECs.

Translation of glycoprotein IIIa in stored blood platelets

BACKGROUND

Platelet (PLT) products have a short shelf life (5 days) owing in part to the deterioration of the quality of PLTs stored at 22 degrees C. This creates significant inventory challenges, and blood banks may suffer shortages and high wastage as a result. The precise biochemical pathways involved in the PLT storage lesion are unknown and must be understood before storage time can be extended.

STUDY DESIGN AND METHODS

Informed by previous proteomics analysis, specific PLT glycoprotein (GP) concentration and surface expression were examined by Western blot and flow cytometry. mRNA concentration was determined by Northern blot and real-time polymerase chain reaction. Protein synthesis was confirmed by [(35)S]methionine labeling.

RESULTS

Western blots of GPIIIa revealed a twofold increase in concentration on Day 7 of storage and a fourfold increase on Day 10. By flow cytometry, surface expression of the GPIIb/IIIa increased by 13.4 percent on Day 7 and 41.9 percent on Day 10. Full-length GPIIIa mRNA was present throughout this storage period and was shown to have a half-life of approximately 2.9 days. Translation of GPIIb and IIIa during storage was confirmed by [(35)S]methionine labeling.

CONCLUSION

This article confirms that PLTs are capable of synthesizing biologically relevant proteins ex vivo throughout a 10-day storage period with particularly long-lived mRNA and provides a framework through which the biochemical mechanisms involved in the translational regulation of proteins thought to be involved in the initiation or exacerbation of the PLT storage lesion can be investigated.

An Integrin-Tetraspanin Interaction Required for Cellular Innate Immune Responses of an Insect, Manduca sexta

In their encounters with foreign intruders, the cells of the insect innate immune system, like those of the mammalian immune system, exhibit both humoral and cell-mediated responses. Some intruders can be dispatched by the humoral immune system alone, but many must be phagocytosed by individual hemocytes or encapsulated by interacting hemocytes. Surface proteins of hemocytes control the abrupt transition of hemocytes from resting, nonadherent cells to activated, adherent cells during these cell-mediated responses. Two of these surface proteins, an integrin and a tetraspanin, interact during this adhesive transition. As demonstrated with a hemocyte adhesion assay and a surface plasmon resonance assay, the large extracellular loop of tetraspanin D76 binds to a hemocyte-specific integrin of Manduca sexta. The interaction between the large extracellular loop domain and hemocyte-specific integrin is interrupted not only by a monoclonal antibody (MS13) that binds to a domain of beta-integrin known to be a ligand-binding site for cell adhesion but also by double-stranded beta-integrin RNA. Transfected S2 cells expressing tetraspanin mediate adhesion of hemocytes. A monoclonal antibody to tetraspanin D76 perturbs the cell-mediated immune response of encapsulation. These studies involving antibody blocking, RNA interference, and binding assays imply a trans interaction of integrin and tetraspanin on hemocyte surfaces.

Does glycosylation of melanoma cells influence their interactions with fibronectin?

Cell surface integrins, especially those binding to fibronectin (FN), participate in processes of tumor cell invasion and metastasis. Changes in glycosylation of cell surface adhesion proteins are often associated with malignant transformation of cells. In this study we examined the influence of swainsonine (SW) on adhesion, wound healing and haptotactic migration on FN, comparing the responses of different human melanoma cell lines: primary WM35 and metastatic WM9, WM239 and A375. We also examined the role of alpha subunits in adhesion to FN. All of the antibodies inhibited adhesion to FN but with different efficiencies depending on the cell line. Adhesion was mediated mainly by integrin alpha(5)beta(1) (WM9, A375), alpha(3)beta(1) (WM35, A375, WM239). Scratch wound repair was significantly faster on FN-coated wells than on plastic for all cells except for WM9. A375 and WM9 had the greatest migration ability, both expressing the highest level of alpha(5)beta(1) integrin. It seems very likely that adhesion to FN can be accomplished by many different integrins, but for effective migration alpha(5)beta(1) integrin is responsible. Only A375 and WM239 cell lines reacted to SW treatment. In the presence of SW WM239 and A375 cells had 70% and 40% increased adhesion to FN, and their migration was decreased 40% and 50%, respectively. Interestingly, although most of the cell lines share a common profile of integrins, each line interacted with FN differently. They differed mainly in the repertoire of integrins used for adhesion, and in the manner in which glycosylation affected these processes. The influence of SW was observed in two metastatic cell lines indicating the contribution of glycosylation status to the progression of melanoma. The lack of reaction to SW in WM9 cells may suggest that there is a threshold in the expression level of the highly branched N-glycans that may influence the adhesion and migration properties of the cell.

E-cadherin endocytosis regulates the activity of Rap1: a traffic light GTPase at the crossroads between cadherin and integrin function

The coordinate modulation of cadherin and integrin functions plays an essential role in fundamental physiological and pathological processes, including morphogenesis and cancer. However, the molecular mechanisms underlying the functional crosstalk between cadherins and integrins are still elusive.

Here, we demonstrate that the small GTPase Rap1, a crucial regulator of the inside-out activation of integrins, is a target for E-cadherin-mediated outside-in signaling. In particular, we show that a strong activation of Rap1 occurs upon adherens junction disassembly that is triggered by E-cadherin internalization and trafficking along the endocytic pathway. By contrast, Rap1 activity is not influenced by integrin outside-in signaling. Furthermore, we demonstrate that the E-cadherin endocytosis-dependent activation of Rap1 is associated with and controlled by an increased Src kinase activity, and is paralleled by the colocalization of Rap1 and E-cadherin at the perinuclear Rab11-positive recycling endosome compartment, and the association of Rap1 with a subset of E-cadherin-catenin complexes that does not contain p120ctn. Conversely, Rap1 activity is suppressed by the formation of E-cadherin-dependent cell-cell junctions as well as by agents that inhibit either Src activity or E-cadherin internalization and intracellular trafficking. Finally, we demonstrate that the E-cadherin endocytosis-dependent activation of Rap1 is associated with and is required for the formation of integrin-based focal adhesions.

Our findings provide the first evidence of an E-cadherin-modulated endosomal signaling pathway involving Rap1, and suggest that cadherins may have a novel modulatory role in integrin adhesive functions by fine-tuning Rap1 activation.

Epithelial membrane protein-2 regulates surface expression of alphavbeta3 integrin in the endometrium

The four-transmembrane protein epithelial membrane protein-2 (EMP2) was recently identified as an endometrial protein necessary for blastocyst implantation, but the mechanism of this role is uncertain. In other cell types, EMP2 controls delivery of certain classes of proteins to the cell surface, including various integrin isoforms (a class of receptors implicated in endometrial-blastocyst interaction). Since alphavbeta3 integrin is an important endometrial molecule involved in blastocyst interaction, we evaluated the role of EMP2 in modulating integrin expression in the HEC1A endometrial cell line and endometrial epithelium in vivo. Elevation of EMP2 expression in HEC1A cells selectively increased the expression of alphavbeta3 integrin on the plasma membrane and was functional as judged by increased cell binding to an alphavbeta3 ligand, fibronectin. Conversely, reduction in EMP2 expression using an EMP2 specific ribozyme decreased the cell alphavbeta3 surface expression. The influence of EMP2 on alphavbeta3 integrin was also observed in vivo as reduction of EMP2 using ribozymes or short hairpin RNA diminished alphavbeta3 integrin expression in glandular and luminal uterine epithelium. Colocalization and coimmunoprecipitation studies suggested that EMP2 and alphavbeta3 integrin predominantly exist in a physically associated state. This study demonstrates for the first time the influence of EMP2 on alphavbeta3 surface expression and suggests that surface trafficking of integrin alphavbeta3 by EMP2 during the window of implantation may be a mechanism for its requirement in endometrial-blastocyst interaction.

Rap1: a turnabout for the crosstalk between cadherins and integrins

The coordinate modulation of the cellular functions of cadherins and integrins plays an essential role in fundamental physiological and pathological processes, including morphogenesis, tissue differentiation and renewal, wound healing, immune surveillance, inflammatory response, tumour progression, and metastasis. However, the molecular mechanisms underlying the fine-balanced relationship between cadherin and integrin functions are still elusive. This review focuses on recent findings on the involvement of the small GTPase Rap1 in the regulation of cadherin- and integrin-dependent cell adhesion and signal transduction. In particular, it highlights some of the novel results recently obtained that raise the possibility of a pivotal role for Rap1 in the functional crosstalk between cadherins and integrins, suggesting interesting new regulatory mechanisms.

Inhibition of endothelial cell migration by thrombospondin-1 type-1 repeats is mediated by beta1 integrins

The anti-angiogenic effect of thrombospondin-1 has been shown to be mediated through binding of the type-1 repeat (TSR) domain to the CD36 transmembrane receptor. We now report that the TSR domain can inhibit VEGF-induced migration in human umbilical vein endothelial cells (HUVEC), cells that lack CD36. Moreover, we identified β₁ integrins as a critical receptor in TSR-mediated inhibition of migration in HUVEC. Using pharmacological inhibitors of downstream VEGF receptor effectors, we found that phosphoinositide 3-kinase (PI3k) was essential for TSR-mediated inhibition of HUVEC migration, but that neither PLCγ nor Akt was necessary for this response. Furthermore, β₁ integrins were critical for TSR-mediated inhibition of microvascular endothelial cells, cells that express CD36. Together, our results indicate that β₁ integrins mediate the anti-migratory effects of TSR through a PI3k-dependent mechanism.

Alpha3beta1 integrin regulates MMP-9 mRNA stability in immortalized keratinocytes: a novel mechanism of integrin-mediated MMP gene expression

Matrix metalloproteinases facilitate cell migration and tumor invasion through their ability to proteolyse the extracellular matrix. The laminin-binding integrin alpha3beta1 is expressed at high levels in squamous cell carcinomas and in normal keratinocytes during cutaneous wound healing. We showed previously that alpha3beta1 is required for MMP-9/gelatinase B secretion in immortalized mouse keratinocytes (MK cells) and that this regulation was acquired as part of the immortalized phenotype, suggesting a possible role for alpha3beta1 during malignant conversion. In the current study, we identify a novel mechanism whereby alpha3beta1 regulates the induction of MMP-9 expression that occurs in response to activation of a MAPK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway. Inhibition of MEK/ERK signaling in wild-type MK cells with a pharmacological inhibitor, U0126, showed that ERK activation was necessary for high levels of endogenous MMP-9 gene expression and activity of a transfected MMP-9 promoter. Furthermore, activation of MEK/ERK signaling in these cells with an oncogenic mutant of Ras, RasV12, increased both endogenous MMP-9 gene expression and MMP-9 promoter activity. Experiments with alpha3beta1-deficient MK cells revealed that alpha3beta1 was required for both baseline levels and RasV12-induced levels of MMP-9 mRNA expression. However, alpha3beta1 was not required for RasV12-mediated activation of ERK or for ERK-dependent MMP-9 promoter activity. Direct comparison of mRNA turnover in the wild type and alpha3-null MK cells identified a requirement for alpha3beta1 in stabilization of MMP-9 mRNA transcripts. These results identify a novel function for integrins in promoting mRNA stability as a mechanism to potentiate MAPK-mediated gene expression. They also suggest a role for alpha3beta1 in maintaining high levels of MMP-9 mRNA expression in response to oncogenic activation of MEK/ERK signaling pathways.

Pro-apoptotic signaling pathway activated by echistatin in GD25 cells

Disintegrins, low molecular weight RGD-containing polypeptides isolated from snake venoms, have seen use as integrin antagonists in the field of tumor biology and angiogenesis. In this study, we investigated the molecular mechanism by which the disintegrin echistatin affects cell adhesion and signaling resulting in an apoptotic response in the GD25 cell system. Wild-type GD25 cells, which lack expression of the beta(1) family of integrin, and stable transfectants expressing the A isoform of beta(1) integrin subunit were used. Nanomolar concentrations of echistatin detached fibronectin- and vitronectin-adherent GD25 cells from immobilized substratum. However, prior to inducing detachment of adherent cells, echistatin caused apoptosis as measured by caspase-3 activation. Either cell detachment or apoptotic response induced by echistatin were more pronounced on fibronectin-adherent GD25 cells than on vitronectin-adherent ones. GD25 cell exposure to echistatin caused a reduction of tyrosine phosphorylation levels of pp125(FAK), whereas it didn't affect either Shc tyrosine phosphorylation levels or Shc-Grb2 functional association. The down-regulation of pp125(FAK) preceded apoptosis and cell detachment induced by echistatin. Our results indicate that pp125(FAK) and not Shc pathway is involved in echistatin-induced apoptotic response in the GD25 cell system.

Identification of Krit1B: a novel alternative splicing isoform of cerebral cavernous malformation gene-1

Cerebral cavernous malformations (CCM) are vascular malformations, mostly located in the central nervous system, which occur in 0.1-0.5% of the population. They are characterized by abnormally enlarged and often leaking capillary cavities without intervening neural parenchyma. Some are clinically silent, whereas others cause seizures, intracerebral haemorrhage or focal neurological deficits. These vascular malformations can arise sporadically or may be inherited as an autosomal dominant condition with incomplete penetrance. At least 45% of families affected with cerebral cavernous malformations harbour a mutation in Krev interaction trapped-1 (Krit1) gene (cerebral cavernous malformation gene-1, CCM1). This gene contains 16 coding exons which encode a 736-amino acid protein containing three ankyrin repeats and a FERM domain. Neither the CCM1 pathogenetic mechanisms nor the function of the Krit1 protein are understood so far, although several hypotheses have been inferred from the predicted consequences of Krit1 mutations as well as from the identification of Krit1 as a binding partner of Rap1A, ICAP1A and microtubules. Here, we report the identification of Krit1B, a novel Krit1 isoform characterized by the alternative splicing of the 15th coding exon. We show that the Krit1B splice isoform is widely expressed in mouse cell lines and tissues, whereas its expression is highly restricted in human. In addition, we developed a real-time PCR strategy to accurately quantify the relative ratio of the two Krit1 alternative transcripts in different tissues, demonstrating a Krit1B/Krit1A ratio up to 20% in mouse thymus, but significantly lower ratios in other tissues. Bioinformatic analysis using exon/gene-prediction, comparative alignment and structure analysis programs supported the existence of Krit1 alternative transcripts lacking the 15th coding exon and showed that the splicing out of this exon occurs outside of potentially important Krit1 structural domains but in a region required for association with Rap1A, suggesting a subtle, yet important effect on the protein function. Our results indicate that maintenance of a proper ratio between Krit1A and Krit1B could be functionally relevant and suggest that the novel Krit1B isoform might expand our understanding of the role of Krit1 in CCM1 pathogenesis.

Human fibroblasts with mutations in COL5A1 and COL3A1 genes do not organize collagens and fibronectin in the extracellular matrix, down-regulate alpha2beta1 integrin, and recruit alphavbeta3 Instead of alpha5beta1 integrin

Dermal fibroblasts derived from types I and IV Ehlers-Danlos syndrome (EDS) patients, carrying mutations in COL5A1 and COL3A1 genes, respectively, synthesize aberrant types V and III collagen (COLL) and show defective organization of these proteins into the extracellular matrix (ECM) and high reduction of their functional receptor, the alpha(2)beta(1) integrin, compared with control fibroblasts. EDS cells also show reduced levels of fibronectin (FN) in the culture medium and lack an FN fibrillar network. Finally, EDS cells prevalently organize alpha(v)beta(3) integrin instead of alpha(5)beta(1) integrin. The alpha(v)beta(3) integrin, distributed on the whole EDS cell surface, shows FN binding and assembly properties when the cells are treated with purified FN. Treatment of EDS cells with purified COLLV or COLLIII, but not with FN, restores the control phenotype (COLL(+), FN(+), alpha(v)beta(3)(-), alpha(5)beta(1)(+), alpha(2)beta(1)(+)). Function-blocking antibodies to COLLV, COLLIII, or alpha(2)beta(1) integrin induce in control fibroblasts an EDS-like phenotype (COLL(-), FN(-), alpha(v)beta(3)(+), alpha(5)beta(1)(-), alpha(2)beta(1)(-)). These results show that in human fibroblasts alpha(2)beta(1) integrin organization and function are controlled by its ligand, and that the alpha(2)beta(1)-COLL interaction, in turn, regulates FN integrin receptor recruitment: high alpha(2)beta(1) integrin levels induce alpha(5)beta(1) integrin organization, while low alpha(2)beta(1) integrin levels lead to alpha(v)beta(3) integrin organization.

Association of sustained ERK activity with integrin beta3 induction during receptor activator of nuclear factor kappaB ligand (RANKL)-directed osteoclast differentiation

Osteoclast differentiation is a multi-step process that involves cell proliferation, commitment, and fusion. Some adhesion molecules, including integrin alphavbeta3, have been shown to have roles in osteoclast fusion. In the course of studying with pharmacologic agents known to inhibit protein tyrosine kinases of the Src family, we found that radicicol increased cell fusion during receptor activator of nuclear factor kappaB ligand (RANKL)-driven differentiation of osteoclasts at concentrations far below the ones shown to inhibit its targets in previous studies. Treatments of low doses of radicicol to RAW 264.7 cells that undergo osteoclastic differentiation in the presence of RANKL enhanced the RANKL-induced gene expression of integrin beta3 without any effect on the expression of integrin alphav, which was constitutively high. The cell surface level of integrin alphavbeta3 complexes was consequently augmented by radicicol. In addition, sustained ERK and MEK activation was observed in cells treated with both radicicol and RANKL. More importantly, modulation of ERK activity by the MEK inhibitor U0126 or the gene transduction of a constitutively active form of MEK resulted in a suppression and increment, respectively, of integrin beta3 induction by RANKL. Our data indicate that sustained ERK activity is associated with integrin beta3 induction and subsequent cell surface expression of the alphavbeta3 integrin complex, which may contribute to cell fusion during RANKL-directed osteoclastogenesis.

RGD modified polymers: biomaterials for stimulated cell adhesion and beyond

Since RGD peptides (R: arginine; G: glycine; D: aspartic acid) have been found to promote cell adhesion in 1984 (Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule, Nature 309 (1984) 30), numerous materials have been RGD functionalized for academic studies or medical applications. This review gives an overview of RGD modified polymers, that have been used for cell adhesion, and provides information about technical aspects of RGD immobilization on polymers. The impacts of RGD peptide surface density, spatial arrangement as well as integrin affinity and selectivity on cell responses like adhesion and migration are discussed.

Opposing roles of Ras/Raf oncogenes and the MEK1/ERK signaling module in regulation of expression and adhesive function of surface transglutaminase

Tissue transglutaminase (tTG) serves as a potent and ubiquitous integrin-associated adhesion co-receptor for fibronectin on the cell surface and affects several key integrin functions. Here we report that in fibroblasts, activated H-Ras and Raf-1 oncogenes decrease biosynthesis, association with beta1 integrins, and surface expression of tTG because of down-regulation of tTG mRNA. In turn, the reduction of surface tTG inhibits adhesion of H-Ras- and Raf-1-transformed cells on fibronectin and, in particular, on its tTG-binding fragment I(6)II(1,2)I(7-9), which does not interact directly with integrins. Analysis of Ras/Raf downstream signaling with specific pharmacological inhibitors reveals that the decrease in tTG expression is mediated by the p38 MAPK, c-Jun NH2-terminal kinase, and phosphatidylinositol 3-kinase pathways. In contrast, increased activation of the ERK pathway by constitutively active MEK1 stimulates tTG mRNA expression, biosynthesis, and surface expression of tTG, whereas MEK inhibitors or dominant negative MEK1 exert an opposite effect. This modulation of surface tTG by ERK signaling alters adhesion of cells on fibronectin and its fragment that binds tTG. Furthermore, transient stimulation of ERK signaling in untransformed fibroblasts by adhesion on fibronectin or growth factors elevates tTG biosynthesis, increases complex formation with beta1 integrins, and raises surface expression of tTG. Finally, ERK activation is required for growth factor-induced redistribution of tTG on the surface of adherent fibroblasts and co-clustering of beta1 integrins and tTG at cell-matrix adhesion contacts. Together, our data indicate that down-regulation of surface tTG by Ras and Raf oncogenes contributes to adhesive deficiency of transformed fibroblasts, whereas stimulation of biosynthesis and surface expression of tTG by the MEK1/ERK module promotes and sustains cell-matrix adhesion of untransformed cells. Contrasting effects of Ras/Raf oncogenes and their immediate downstream signaling module, MEK1/ERK, on tTG expression are consistent with adhesive function of surface tTG.

The fibronectin-binding integrins alpha5beta1 and alphavbeta3 differentially modulate RhoA-GTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis

We have studied the formation of different types of cell matrix adhesions in cells that bind to fibronectin via either alpha5beta1 or alphavbeta3. In both cases, cell adhesion to fibronectin leads to a rapid decrease in RhoA activity. However, alpha5beta1 but not alphavbeta3 supports high levels of RhoA activity at later stages of cell spreading, which are associated with a translocation of focal contacts to peripheral cell protrusions, recruitment of tensin into fibrillar adhesions, and fibronectin fibrillogenesis. Expression of an activated mutant of RhoA stimulates alphavbeta3-mediated fibrillogenesis. Despite the fact that alpha5beta1-mediated adhesion to the central cell-binding domain of fibronectin supports activation of RhoA, other regions of fibronectin are required for the development of alpha5beta1-mediated but not alphavbeta3-mediated focal contacts. Using chimeras of beta1 and beta3 subunits, we find that the extracellular domain of beta1 controls RhoA activity. By expressing both beta1 and beta3 at high levels, we show that beta1-mediated control of the levels of beta3 is important for the distribution of focal contacts. Our findings demonstrate that the pattern of fibronectin receptors expressed on a cell dictates the ability of fibronectin to stimulate RhoA-mediated organization of cell matrix adhesions.

Fas activation induces renal tubular epithelial cell beta 8 integrin expression and function in the absence of apoptosis

Cell fate following Fas (CD95) ligand or agonistic anti-Fas antibody stimulation is determined by multiple factors, including Fas expression level, microdomain localization, and modulating cytokines. Highly expressed Fas clusters and activates a canonical apoptosis signaling pathway. In less susceptible cells, Fas transduces apoptosis-independent signals, which are not well defined, but have been linked to inflammation, angiogenesis, and fibrosis. To identify apoptosis-independent Fas pathways, cultured renal tubular epithelial cells were stimulated with agonistic anti-Fas antibodies under conditions that did not cause cell death. Analysis of filter cDNA microarrays revealed beta(8) integrin subunit mRNA induction in Fas-stimulated cells. beta(8) integrin mRNA expression increased within 3-6 h of Fas ligation due to enhanced mRNA stabilization, and mRNA increases were sustained for 48-72 h. Expression of plasma membrane beta(8) integrin, as well as its heterodimer partner alpha(v), was increased by Fas activation with a similar kinetic pattern. Fas-induced alpha(v)beta(8) expression correlated with increased migration to vitronectin, the ligand for alpha(v)beta(8). Results from studies with function-blocking antibodies against other alpha(v)beta integrins or suppression of beta(8) integrin expression by RNA interference demonstrated that induced beta(8) integrin expression mediated Fas-stimulated migration. We conclude that alpha(v)beta(8) integrin induction defines an unexpected role for Fas in cell migration, rather than as a cell death receptor.